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Geology
本研究基于薄断面观测、高压注汞、低温氮素吸附和扫描电镜观测，探讨了准噶尔盆地南缘西山尧组煤测砂岩储集层物性、孔隙类型、成岩特征及储层质量控制效应。结果表明，砂岩的孔隙度和渗透率一般较低，具有中高质地质成熟度和低成分成熟度。砂岩储集空间主要由残余的晶间孔隙、次生溶出孔、晶间孔隙和微裂缝组成。测煤砂岩成岩阶段处于介生阶段A1-A2阶段，其成岩相互作用类型主要包括压实、胶结和溶蚀。煤的储层质量测量砂岩由于高基质含量和塑料碎片含量而因压实而恶化。由于煤测量烃源岩的热演化过程中会产生大量的有机酸，煤测量砂岩遭受强烈的溶解。
- tight sandstone
- coal measure
- southern margin of Junggar basin
- diagenetic process
- reservoir quality control
- reservoir forming
1. 引言
准噶尔盆地煤层气资源丰富，其中埋藏深度小于2000米的煤层气预测资源为3.11×1012m3，准噶尔盆地（SJB）的南缘对煤层气具有最佳的保存条件[1]。SJB主要含煤地层为中侏罗纪下部，沉积环境主要包括冲积扇、扇形三角洲、辫状河流三角洲和岸边浅湖[1，2，3]。SJB具有厚厚多煤层的特点，横向分布广泛，纵向层层叠层砂岩、页岩、煤层频繁，有利于煤计量气的共积[4，5，6，7，8，9，10]（CMG，包括煤层气，煤测量致密砂岩气和煤测量页岩气）。鄂尔多斯盆地成功开发的先例[5，11，12]和CMG共生积累理论的成熟[7，8]使业界意识到CMG发展的巨大潜力。储集层质量对煤量致密砂岩气的高效开发至关重要，成岩环境和构造事件的区域差异导致影响储集层质量的不同因素[13，14，15，16，17，18，19]。为了有效开发SJB煤措施中所含的丰富致密砂岩，有必要对煤措施砂岩的岩石学特征和储集层质量进行研究。
致密砂岩储集层通常经历复杂的成岩过程，导致埋藏和热演化过程中逐渐致密[13，14，15，16，17，18]。储集层质量的影响包括沉积、成岩作用和表观成因，具体影响因素包括碎屑岩组成、沉积环境、沉积相、埋藏温度和压力[19，20]。沉淀控制颗粒的大小、分选和排列，以及矿物转化和沉积结构[16，21，22，23]。成岩作用对砂岩的影响主要表现在温度、pH值、压力等方面，表现为压实、溶解、胶结、变质[19，21，24]。压实是导致孔隙率和渗透率降低的主要因素之一[25]，储层质量尤其受到压实程度和塑料颗粒（如碎石碎片、云母、泥质碎片）和基质含量的影响[19，26]。胶结一般会导致孔喉堵塞，严重降低渗透率[27、28、29、30、31，32]，但同时也能在一定程度上抑制储集层质量的恶化：粘土矿物胶结形成的晶间孔隙可以为砂岩提供储物空间[31，33，34]];覆盖颗粒表面的粘土膜抵抗压实并抑制其他类型的水泥的发展[13，35]，从而保留残留的孔隙;粘土膜覆盖颗粒表面，可以抵抗压实，抑制其他类型的水泥的发展;渗氮中的石英胶结和碳酸盐胶结可以提高砂岩的抗压实能力[15，36，37]。通过分析煤测砂岩的沉积环境、组成和成岩特性，可以阐明西山尧组煤测砂岩储集层质量控制因素，解释煤测砂岩储集层的形成和演化过程。
准噶尔盆地南缘西山瑶组煤层以中低煤层为主，砂岩、泥岩、煤层交织频繁[1]。煤测量砂岩的成岩作用受到煤层和深色页岩热演化的强烈影响[7，8]。以往砂岩储集层的研究主要集中在孔喉形成机理、孔喉定量表征、盆地内不同区间或层层砂岩储集层油气侵入与成岩作用的关系[38，39，40，41，42，43]].然而，对中侏罗统西山尧组含煤砂岩的研究也集中在沉积环境的演化、储集层类型的分类、储集层的分类与评价等方面[2，4，44，45，46，47]。由于对西山瑶组煤测砂岩储集层的成岩层序了解，且对储集层质量的影响不够全面，亟待开展相关工作。本文结合浇注薄片观察、扫描电镜（SEM）、XRD实验、高压汞侵入、低温氮气吸附、煤测量页岩Ro试验等技术，对以下内容进行研究：（1）确定西山尧组砂岩的特性;（2）明确了西山尧组煤测砂岩的成岩演化序列;（3）明确了沉积和成岩作用对煤测量砂岩储集层质量的控制作用。本研究有望为西山尧组煤措施致密砂岩气的勘探开发提供参考。
2. 地质背景
准噶尔盆地位于新疆北部;它是一个大型的中生代- 新生代坳陷盆地，面积约为13.4×104公里2平面呈近似三角形，东西宽1120公里，南北宽800公里[1，2，3]。自古生代晚期以来，准噶尔盆地经历了海西、印度支那、燕山和喜马拉雅的构造运动，多相应力场的叠加导致了其结构复杂性[2，44，48]。准噶尔盆地可分为六个构造单元：吕梁隆起、乌伦古洼地、北天山皮埃蒙特冲断带、西部隆起、中央洼地和东部隆起[3，49]（图1A）。SJB位于天山北部的皮埃蒙特推力带，东西方向伸长，南北宽近200公里（图1B）。天山北皮埃蒙特推力带是多相叠加和继承构造带，通常分为五个次级构造单元，包括四州凹陷、齐固断裂褶皱带、火马图背斜带、桓斜带、阜康断裂带[50]。
SJB表面的主要部分被第四纪地层覆盖。SJB地区的地层从旧到新是二叠纪，三叠纪，侏罗纪和古新世（图1C）。其中，八道湾组和西山窑组是研究区的主要含煤地层，广泛分布在SJB（图1C）。西山尧组的含煤地层形成于湖相-三角洲沉积系统[2，3]。西山尧组地层在今霍尔果斯河和四公河之间有多个煤堆积中心，厚大于40 m，受构造和沉积条件的控制[1]。
3. 煤测量砂岩成岩成岩的副成因序列
3.1. 成岩阶段的测定
成岩阶段的澄清应基于古温（T）、镜质体反射率（Ro）、最大热解峰温度（Tmax）和I/S的蒙脱石混合层比（S%）、自生矿物的空间和时间组合以及颗粒之间的接触关系[5，22，67]。研究区西山尧组煤测页岩Ro为0.63~0.89%。煤测量砂岩的粘土矿物以I/S、高岭石和伊利石为主，I/S的混合层比（S%）为10-30%（平均为18%）。压实对煤测量砂岩有很大影响，其接触关系由线接触、凹凸接触和缝合接触组成。Wang等人[68]进行的侏罗纪地层热学重建[68]发现，西山瑶组的最大埋藏深度超过3000米，古温度超过100°C。 根据碎屑岩成岩阶段分类标准（我国油气行业标准SY/T5477-2003），可以综合确定西山尧组煤测砂岩成岩阶段处于介生系A1和A2阶段（图8）。
3.2. 煤测量砂岩的成岩序列
结合SJB的热学和埋藏史，参考我国油气行业标准SY/T5477-2003中碎屑岩成岩阶段的划分，总结了该区西山尧组煤测量砂岩的成岩演化情况如下：
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同源
在这个阶段，成岩环境尚未与上覆水完全断开。沉积在泥炭沼泽中的有机物产生一氧化碳2在微生物的作用下溶解在水中，使成岩环境呈弱酸性。由于沉积水的酸性，碎屑颗粒的表面很少被亚氯酸盐粘土膜覆盖。由于沉积期水动力环境较强，煤测砂岩中含有一定量的碳酸盐塑性硅矿，碳质碎片会掺入砂岩中。
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Eodiagenesis（在深度<2公里，温度<70°C，Ro<0.5%）
发育阶段成岩作用的类型和程度将影响成岩作用的类型和后期成岩期储集层的物理性质[22]。在这个阶段，成岩环境是弱酸性的，成岩类型包括机械压实，胶结（主要是泥质胶结）和溶解，成岩环境逐渐转变为封闭系统。压实显著降低了初级孔隙率[69，70]，这是由于埋藏深度较浅且沉降率高。岩石处于弱固结和半固结状态，碎屑颗粒的接触关系从点接触到点线接触发生变化。该阶段测煤砂岩的孔隙类型主要是初级孔隙，少量的硅藻土碳酸盐水泥填充孔隙。
随着气温的升高，煤层和深色页岩逐渐释放出有机酸和一氧化碳。2 [[64]，这使得成岩环境逐渐转变为弱酸。长石和碎屑颗粒开始轻微溶解并形成溶解孔（图9）。
Figure 9. The paragenetic sequence and pore evolution of the Xishanyao formation in the SJB. The thermal evolution history is based on the study of Wang et al. [68] with some modifications. Solid blue lines represent probable timing based on observed diagenetic and mineralization phases. While the dashed blue lines represent inferred or not well-constrained diagenetic and mineralization phases. J2x = Xishanyao formation; I/S = illite/smectite; Ro = vitrinite reflectance.
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Mesodiagenesis (at the depth > 2 km, temperature > 70 °C, and Ro > 0.5%)
The effect of compaction in the mesodiagenesis A stage was extremely strong, and the compaction type gradually shifted from mechanical compaction to chemical compaction. The contact relationships change from point contact and line contact to line contact, concave–convex contact and suture contact during this stage, along with a large proportion of brittle minerals broken up. The dissolution pores formed in the eodiagenesis stage are damaged under the compaction, and the damage of compaction to the quality of the reservoir is further strengthened. The mesodiagenesis A stage can be further divided into two substages, A1 and A2, according to temperature and Ro. These two substages have certain differences in diagenetic environment and diagenetic reaction:
In the mesodiagenesis A1 stage, the temperature was 70–90 °C, and the Ro was 0.5–0.7%, in an acidic diagenetic environment. The amount of organic acid generated in coal measures reaches the highest level at this temperature [71], the diagenetic environment is acidic, and the tensity of dissolution reached the highest level (Figure 9). The massive dissolution of feldspar, aluminosilicate lithic fragments and early micrite carbonate cement formed a large number of dissolution pores and mold pores. Because of the relatively closed diagenetic system, the dissolution products cannot be discharged. This has led to the kaolinite filling pores and quartz overgrowth (part of the SiO2 comes from the chemical compaction).
In the Mesodiagenesis A2 stage, the temperature was 90–130 °C, and the Ro was 0.7–1.3%. The acidic diagenetic environment is gradually weakened due to the decomposition of organic acids. The effect of dissolution decreases gradually, while the cementation is gradually increases. The quartz overgrowth gradually decreases, which is manifested as second level quartz cementation. Late period carbonate cementation (dolomite and ankerite) gradually filled the pore space (Figure 9). The clay minerals appear abundantly and fill the pores, further reducing the porosity.
4. Diagenetic Control of Coal Measure Rvoir Quality
Reservoir quality is influenced by diagenetic activities, such as compaction, cementation, dissolution, recrystallization, and metasomatism [20,72]. For the Xishanyao formation coal measure sandstone, compaction, dissolution and cementation are the main diagenetic controlling factors for reservoir quality.
4.1. The Influence of Compaction on Reservoir Quality
The compaction is one of the main factors that cause the reduction of intergranular pore volume [73] and the densification of coal measure sandstone reservoirs. Plastic particles in the sandstone have a weak compaction resistance, while the rigid particles have a high compaction resistance ability [61,74]. The coal measure sandstones of the Xishanyao formation contain a high content of plastic grains, such as volcanic lithic fragments, muddy gravel and carbonaceous fragments, which makes its compaction resistance ability weak.
The coal measure sandstone reservoirs are strongly affected by mechanical compaction, which is characteristic of the close contacted clastic particles, the ruptured rigid particles, and the deformed plastic particles. Chemical compaction commonly happens in SJB due to the high intensity of compaction, which may lead to particle suture contact and reduced interparticle space. The intense mechanical and chemical compaction has a great negative effect on the physical properties of the sandstone reservoir.
4.2. The Influence of Cementation on Reservoir Quality
The coal measure sandstone of the Xishanyao formation in the SJB is mainly clay mineral cement, followed by quartz cement and carbonate cement. The development of these cementations is one of the main factors leading to the significant decline in reservoir quality [74,75,76,77].
The closed diagenetic system of coal measure sandstone prevents the dissolution products from being discharged in time, which also leads to the development of authigenic kaolinite cementation and quartz cementation. The developed kaolinite cement fills the pore space in large quantities, but its loose inter-crystalline structure contains inter-crystalline pores that can resist compaction [78]. In addition, illite and I/S mainly fill intergranular pores and pore throats, thereby reducing the reservoir quality of the sandstone [65,79,80]. In summary, different types of clay minerals occur widely and fill or divide the pore space, resulting in tortuous pore throats, reducing pore connectivity, and weakening seepage capacity [13,37,65,79,80].
The content of volcanic lithic fragments in coal measure sandstone is relatively high, which may inhibit the quartz overgrowth to some extent [77]. It can be observed that the overgrowth level of the coal measure sandstone is shown in the second stage (Figure 5A,C). The quartz overgrowth in the eodiagenesis stage can help the sandstone better resist compaction. In the mesodiagenesis, pressure solution, feldspar dissolution and clay mineral transformation produced many SiO2 (aq), which developed into the quartz cement [13,81,82].
The carbonate cement is locally developed in coal measure sandstones. In the eodiagenesis stage, several micrite carbonate particles blended in the coal measure sandstone, which blocked and filled the intergranular pores. In the mesodiagenesis A2 stage, the late period dolomite and ankerite cements appeared in the sandstone, which blocked the intergranular pores of the sandstone and reduced the reservoir quality (Figure 5A,H,I).
4.3. The Influence of Dissolution on Reservoir Quality
The dissolution in diagenesis has a positive effect on reservoir physical properties [13]. The coal seam and dark shale begin to produce organic acids through the decomposition of plant residues during the eodiagenesis stage [71] and reaching a peak in the mesodiagenesis stage (temperature range from 80 °C to 140 °C, [37,83]). The amount of organic acid generated in the coal measure is much higher than in other source rocks [5,7,84].
The sedimentary environment makes the sandstone have good primary physical properties and pore connectivity, and the weak dissolution in the eodiagenesis stage provides a seepage channel for the migration of acidic fluids in the mesodiagenesis stage. The large proportion of aluminosilicate minerals, such as feldspar and volcanic in coal measure sandstone provides a good material basis for the dissolution [5]. Dissolution is most intense in mesodiagenesis. During this period, the thermal evolution of organic matter releases a large number of organic acids into the diagenetic system of the sandstone, causing the soluble minerals to selectively dissolve and form a large number of intragranular dissolved pores and intergranular dissolution pores [35]. Overall, the diagenetic stage of the coal measure sandstone in this area is mainly in the A1 and A2 stages of the mesodiagenesis, which happens to be the most intense stage of organic acid dissolution. The dissolution has brought a great positive effect on the physical properties of the reservoir.
5. The Influence of Sedimentation on the Reservoir Quality of Coal Measure Sandstone
The influence of the sedimentation on reservoir quality cannot be well evaluated only by sedimentary facies, which need to be analyzed from the aspects of composition, texture, and diagenetic alteration [19,72,89,90,91].
The parameters, such as the average particle size, sorting, roundness, and matrix content of the sandstone determine the initial intergranular volume [15,21,32,92,93], and the initial porosity of sediment can reach 40% [94]. Permeability is a function of the matrix and grain size [39,95]. For sandstones in the same diagenetic stage, the larger the average grain size, the better the sorting and roundness; the lower the matrix content, the stronger the compaction resistance, and the better the physical properties of the reservoir will be [96,97]. Reservoirs with good physical properties are conducive to the exchange of fluids, thereby enhancing the strength of cementation and dissolution during diagenesis [67,98].
The coal measures sandstone in the Xishanyao formation is formed in a lacustrine-braided river delta environment [1,3]. The sandstone particles are poor to medium sorted, sub-angular to sub-round roundness, have high matrix content, and have a medium-high texture maturity. Coal measure sandstones with medium-high texture maturity are prone to grain rearrangement (rotation and slippage) during compaction, thereby enhancing the effect of compaction [99].
Differences in the composition of particles lead to differences in physical and chemical diagenesis reactions during diagenesis [67,98,100]. When the quartz content is low, the reservoir quality increases with the increase in the quartz content. If the content of detrital quartz is higher than 75%, the lower content of feldspar will lead to the reduction of dissolved pores, and at the same time, the quartz cementation in the reservoir will be more developed, thereby reducing the porosity and permeability of the reservoir [101,102].
The compositional maturity of coal measure sandstone in the SJB is relatively low, and its provenance is composed of pyroclastic rocks, intermediate-acid magmatic rocks, and metamorphic rocks derived from the Tianshan Mountains [46,54]. Because the provenance is closer to the depositional area, the proportion of feldspar and volcanic lithic fragments in the coal measure sandstone is relatively large. Due to the weak compaction resistance of feldspar, lithic fragments and plastic particles (such as muddy gravel and carbonaceous fragments) [103,104], the coal measure sandstone reservoirs are greatly affected by mechanical compaction. In addition, the abundant organic matter and the high content of feldspar and lithic fragments in the coal measure strata also lead to strong dissolution during the diagenetic stage.
Due to the warm and humid environment during the deposition period of the Xishanyao formation, the coal measures are generally developed. The abundant organic matter generated CO2 under the action of microbial fermentation and made the water weakly acidic [9]. The weakly acidic water medium condition makes it difficult to coat clay minerals on the grain surface and form chlorite clay films. The absence of the clay film on the grain surface makes the reservoir easily affected by compaction and makes the observation of quartz overgrowth more difficult. In addition to the dissolution during the diagenesis, a part of kaolinite is also formed during the depositional period, and their origins were mainly because of the mineral transformation, illite and chlorite under acidic conditions, and feldspar alteration [5,65,85].
This entry is adapted from the peer-reviewed paper 10.3390/en15155499
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